1. Many arthropods depend on carbohydrate-rich food sources such as nectar or honeydew. Nevertheless, we often know little about the extent to which various sugar sources contribute to the diet of arthropods. 2. One way to study food use in the field is to analyse guts of collected insects for source-specific compounds. Sugar sources often show distinct differences in their carbohydrate composition. This applies especially to honeydew, the excretion product of phloem-feeding ‘Sternorrhynchae’, which often features a broad range of phloem-feeder synthesized di- and oligosaccharides. 3. Out of these oligosaccharides, melezitose, has been widely used as an indicator of honeydew consumption. The use of melezitose or other honeydew saccharides as ‘signature sugars’ hinges on the assumption that the production of these sugars is unique to honeydew-secreting insects. 4. Here we show that the hymenopteran parasitoids Diadegma semiclausum and D. insulare synthesize the trisaccharides melezitose and erlose as well as the disaccharide maltose when fed sucrose, but not when feeding on an equimolar glucose–fructose mixture. 5. The presence of melezitose, erlose and maltose was confined to the parasitoid's digestive tract, indicating that enzyme activity is restricted to this area. D. semiclausum excrement contained low overall sugar concentrations and low relative levels of melezitose, erlose and maltose. 6. Possible functions of sugar synthesis in these nectar-feeding insects are discussed.